Global Drainage Systems
Senior 6 · Geography
Global Drainage Systems
Lesson 2 of 5
25 min
At Rusumo, on the border with Tanzania, the Akagera drops over a band of hard rock in a roaring fall and then races through a narrow gorge. A hundred kilometres upstream, the Nyabarongo winds slowly across a flat, marshy valley floor, looping back on itself. These are not two kinds of river but two stages of the same kind: a river's landforms change from source to mouth because its gradient, its energy and its main process change. The American geographer W. M. Davis (1899) described three stages, youthful, mature and old, a scheme still used to organise river landforms.
Definition
Long profile: a section along the course of a river from source to mouth, showing how its gradient changes. Cross-profile: a section across the valley at one point.
Graded profile: a smooth, concave long profile in which the river's energy just balances the load it must carry, so that it neither erodes nor deposits overall.
Knickpoint: a sudden break of slope on the long profile, often marked by a waterfall or rapids.
Gradient: the fall in height divided by the horizontal distance, often given in m per km or as a ratio such as 1 in 200.
Sinuosity index: channel length divided by the straight-line distance between the same two points; a value above 1.5 is usually called meandering.
Figure 1: the concave long profile and typical cross-profiles. In the upper course vertical erosion dominates; in the middle course lateral erosion widens the valley; in the lower course deposition builds a floodplain.
Worked example 1: gradient
Problem. An illustrative river falls from 2,300 m at its source to 1,400 m at a lake 180 km away. In its first 12 km it falls 600 m. Compare the overall gradient with that of the first 12 km.
Solution.
The upper course is ten times as steep as the average, as expected on a concave profile. Notice that the river ends at a lake, not the sea, so its base level here is the lake surface.
Worked example 2: sinuosity
Problem. On a map, a river channel measures 14.2 km between two bridges that are 5.0 km apart in a straight line. Calculate its sinuosity index and describe the channel.
Solution.
Since 2.84 is well above 1.5, the channel is strongly meandering, typical of a mature or old river on a gentle gradient.
| Landform | How it forms |
|---|---|
| V-shaped valley | Vertical erosion deepens the channel, while weathering and mass movement wear the valley sides back into a V |
| Interlocking spurs | The river lacks the energy to cut through hard spurs of land, so it winds around them; seen from upstream, the spurs overlap like the teeth of a zip |
| Waterfall | Where a band of hard rock lies over softer rock, the soft rock erodes faster; the falling water and its load scour a plunge pool and undercut the hard rock, leaving an overhang that eventually collapses; the fall retreats upstream |
| Gorge | A steep-sided, narrow valley left as a waterfall retreats upstream, such as the Batoka Gorge below Victoria Falls on the Zambezi |
| Rapids | Turbulent, fast water over alternating bands of hard and soft rock, a series of small steps |
| Potholes | Circular hollows drilled into the rock bed by pebbles swirled round in eddies (abrasion) |
As the gradient falls, the river swings from side to side and lateral erosion becomes more important than vertical erosion. The key landform is the meander. Water flowing round a bend is thrown towards the outer bank, so the line of fastest, deepest flow (the thalweg) swings to the outside. There, hydraulic action and abrasion undercut the bank to form a river cliff. On the inside of the bend the water is slow and shallow, and the river deposits sand and gravel as a point bar or slip-off slope. A corkscrew-like helicoidal flow carries material from the outer bank to the inner bank of the next bend. As meanders migrate sideways and downstream, they trim back the spurs and widen the valley floor.
Figure 2: a meander in plan and in cross-section. The asymmetric cross-section, deep under the river cliff and shallow over the point bar, is the evidence of erosion on one bank and deposition on the other.
| Landform | How it forms |
|---|---|
| Floodplain | A wide, flat valley floor built from layers of alluvium (silt and sand) deposited when the river floods, and widened by migrating meanders |
| Levees | Raised banks along the channel: when a river overflows, it loses energy at once and drops its coarsest load right beside the channel; repeated floods build the banks up |
| Ox-bow lake | Erosion on the outer banks narrows the neck of a meander until, during a flood, the river cuts through it; deposition seals off the old loop, which is left as a horseshoe-shaped lake that slowly fills to a meander scar |
| Deferred tributary | A tributary that cannot cross the levees and runs parallel to the main river for some distance before joining it |
| Braided channel | Where a river carries much coarse load and its discharge varies, it deposits bars that split the flow into many interlacing channels |
| Delta | Where a river enters a calm sea or lake, it slows and deposits its load faster than waves and currents remove it, building land outward through distributaries; deltas may be arcuate (fan-shaped: the Nile, the Niger), bird's-foot (long fingers along distributaries: the Mississippi) or cuspate (pointed, shaped by waves from two sides: the Tiber) |
| Estuary | A drowned river mouth where fresh and salt water mix and tides move silt in and out, leaving mudflats and salt marshes; the Thames and the Congo end in estuaries |
Warning
Common mistakes
Tip
Exam technique. "Explain the formation of…" questions need a sequence: conditions, process, result, and a labelled diagram. For a waterfall: hard rock over soft rock → undercutting and plunge pool → overhang collapses → retreat and gorge.
Geographer Online: "Meanders and Ox Bow Lakes – diagram and explanation". Follow how the thalweg causes erosion on one bank and deposition on the other, and compare the diagrams with Figure 2.
Practice
Summary
Sources and credits
Photos: RusumoFalls1 (SteveRwanda, CC BY-SA 3.0), Nyabarongo (Fanny Schertzer, CC BY-SA 3.0) and Nile River and delta from orbit (Jacques Descloitres, MODIS Rapid Response Team, NASA, public domain), via Wikimedia Commons.
Video: "Meanders and Ox Bow Lakes – diagram and explanation", Geographer Online, embedded from YouTube.
Figures 1 and 2, the worked examples and the exercises were drawn and written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
